In general, a pressure detection device is known which is fitted to an engine mounted in an automobile and which uses a piezoelectric element (sensor element) incorporated to detect pressure (combustion pressure) within a combustion chamber, and the pressure detection device generally has a structure as shown in FIG. 6 and is fitted to the engine. In FIG. 6, symbol 601 represents an automobile engine, symbol 602 represents a pressure detection portion, symbol 603 represents a wiring cable and symbol 604 represents a control board. In this case, the pressure detection portion 602 is connected to the control board 604 through the wiring cable 603 which includes a power supply line, a GND line and a signal line for transmitting a detection signal. The control board 604 has the function of performing restraint on the engine 601 based on the detection signal from the pressure detection portion 602. On the other hand, the pressure detection portion 602 generally uses a piezoelectric crystal as a sensor element for detecting pressure, and feeds a charge signal corresponding to the amount of variation in the pressure obtained from the piezoelectric crystal to a signal processing circuit, and the charge signal is integrated by an integral circuit and is thereby converted into a voltage (detection signal) similar to the pressure.
Conventionally, as this type of signal processing circuit, a signal processing device of a piezoelectric sensor disclosed in patent literature 1 is known. FIG. 7 shows an outline of the signal processing device disclosed in patent literature 1. In FIG. 7, one terminal of a sensor element 701 is connected through a capacitor 702 to the inverting input terminal of an operational amplifier 703, and the other terminal of the sensor element 701 is connected to the GND (ground) of the circuit. The non-inverting input terminal of the operational amplifier 703 is connected to a reference power supply 704, a reference voltage Vr is fed, and the inverting input terminal and the output terminal of the operational amplifier 703 are connected with a parallel circuit of a charge capacitor 705 and a discharge resistor 706 of a high resistance value. Furthermore, a signal output from the operational amplifier 703 is output through an operational amplifier 707 as a signal voltage Vout to the outside. FIG. 8(a) illustrates the waveform of the amount of charge (charge signal) Qi obtained from the sensor element 701, and FIG. 8(b) shows an example of the signal voltage Vout obtained from the operational amplifier 707. The voltage waveform shown in FIG. 8(b) is a waveform which is obtained by integrating the amount of charge Qi shown in FIG. 8(a). Although the base voltage VO (signal voltage when the amount of charge Qi is 0) of the signal voltage Vout is biased by the reference power supply 704, it does not match the reference voltage Vr and is lower than the reference voltage Vr due to a so-called waveform sinking phenomenon caused by the charge capacitor 705.
Incidentally, in terms of ensuring a satisfactory size and reliability, it is difficult for the control board 604 described previously to be incorporated in the pressure detection portion 602, and in general, the control board 604 is arranged in another place within an engine room and is connected with a wiring cable 603. Hence, the wiring cable 603 is drawn within the engine room, and since the engine room is often exposed to harsh environments due to temperature, humidity, wind pressure and the like, and external stress may be applied thereto, when the engine room is exposed to such environments for a long period of time, the wiring cable 603 may disconnect.
Hence, in terms of ensuring the safety and reliability of an automobile, a means to detect disconnections which reliably and rapidly detects this type of disconnection is needed, and in general, a disconnection detection circuit 130 as shown in FIG. 9 is provided. In FIG. 9, symbol 602 represents the pressure detection portion shown in FIG. 6, symbol 604 represents the control board shown in FIG. 6 and symbol 603 represents the wiring cable shown in FIG. 6. Symbol 101 represents the part of a signal processing device (signal processing circuit) which includes the sensor element shown in FIG. 7. Symbol 102 represents an output amplifier included in the pressure detection portion 602, and symbol 110 represents an input amplifier included in the control board 604. As the elements of an abnormal detection circuit 130, a pull-up resistor 103 which pulls up the output of the output amplifier 102 is included, and a pull-down resistor 109 which pulls down the input of the input amplifier 110 is also included. Symbols 106 and 108 represent wiring cable sockets.
If either the power supply line 112, the GND line 111 or the signal line 113 included in the wiring cable 603 disconnects, it can be detected with the abnormal detection circuit 130. Specifically, when the signal line 113 disconnects, the input of the control board 604 is fixed by the pull-down resistor 109 to the voltage level of the GND. If the power supply line 112 disconnects, since no power is supplied to the side of the pressure detection portion 602, the output of the output amplifier 102 is the voltage level of the GND or has a high impedance, and the input of the control board 604 is fixed by the pull-down resistor 109 to the voltage level of the GND. Furthermore, if the GND line 111 disconnects, the output of the output amplifier 102 is the power supply voltage level or has a high impedance, and when the output of the output amplifier 102 is the power supply voltage level, the input of the control board 604 is fixed to the power supply voltage level whereas when the output of the output amplifier 102 has a high impedance, the input of the control board 604 is fixed by the pull-up resistor 103 and the pull-down resistor 109 to a voltage obtained by dividing the power supply voltage Vdd. Here, when the resistance value of the pull-up resistor 103 is set significantly lower than the resistance value of the pull-down resistor 109, the output voltage serving as a detection signal is a fixed voltage in the vicinity of the power supply voltage Vdd. As described above, if either the power supply line 112, the GND line 111 or the signal line 113 in the wiring cable 603 disconnects, the input signal of the control board 604 is fixed to the voltage level of the GND or the power supply voltage level (including the level in the vicinity thereof), with the result that based on this, the side of the control board 604 can detect the disconnection. The same function can be achieved even when the output on the side of the pressure detection portion 602 is pulled down and the input on the side of the control board 604 is pulled up.